Image forming apparatuses such as a printer and copying machine have recently been developed for color printing. As a fixing unit used in such a color image forming apparatus, a heat roller type fixing unit having an elastic layer on a fixing member is known well.
The heat roller type fixing unit having an elastic layer suffers a large heat capacity of the heat roller itself, and thus a long time (warm-up time) necessary to raise the fixing roller to a temperature suitable for fixing a toner image. In addition, the cost of the fixing unit rises.
As a fixing unit having a short warm-up time, a belt fixing type fixing unit often used in a monochrome image forming apparatus is known well. FIG. 19 depicts a view showing an example of a belt fixing unit 201.
A fixing belt unit 202 is an assembly having a heater holder 207 with an almost semicircular troughed cross section, a fixing heater 204 which is fixed on the lower surface of the heater holder 207 along the longitudinal direction (direction perpendicular to the sheet surface of FIG. 19) of the heater holder, and a fixing belt 203 of an endless belt-like (cylindrical) thin layer which is loosely fitted on the heater holder 207 having the fixing heater.
Reference numeral 205 denotes an elastic press roller whose core is freely rotatably born at its two ends between the side plates of the fixing unit 201.
In the fixing belt unit 202, the fixing heater 204 is arranged above the elastic press roller 205 so that the fixing heater 204 faces down and becomes parallel to the press roller 205. The two ends of the heater holder 207 are pressed down by a biasing means (not shown) at a predetermined press force. The lower surface of the fixing heater 204 is pressed against the elastic force of the press roller 205 to the upper surface of the elastic press roller 205 via the fixing belt 203, forming a fixing nip 206 with a predetermined width.
The elastic press roller 205 is driven to rotate by a driving mechanism (not shown) at a predetermined peripheral speed in a direction indicated by an arrow. By rotational driving of the elastic press roller 205, the rotational force acts on the fixing belt 203 by the frictional force between the elastic press roller 205 and the outer surface of the fixing belt 203 at the fixing nip 206. While the fixing belt 203 slides with its inner surface in tight contact with the lower surface of the fixing heater 204 at the fixing nip 206, the fixing belt 203 is driven to rotate around the heater holder 207 at a peripheral speed substantially corresponding to that of the elastic press roller 205 in a direction indicated by an arrow.
The fixing belt 203 is an endless belt of a heat-resistant resin about 50 μm thick, and its surface is covered with a 10-μm thick mold release layer (coating layer of a fluoroplastic or the like). In order to reduce the heat capacity of the fixing belt 203, the fixing belt 203 does not use any elastic layer.
The fixing heater 204 is prepared by forming a resistance heating element on a ceramic substrate. A temperature detection unit 209 abuts against the back surface of the fixing heater 204, and detects the temperature of the fixing heater 204. In accordance with the detected temperature, a control unit (not shown) controls power supply to the fixing heater 204 so as to set the temperature of the fixing heater 204 to a desired one.
The elastic press roller 205 is driven to rotate, the fixing belt 203 is driven to rotate along with this, and the fixing heater 204 reaches a predetermined temperature. In this temperature-controlled state, a print medium P bearing an unfixed toner image t is introduced between the fixing belt 203 and the elastic press roller 205 at the fixing nip 206. The unfixed toner image bearing surface of the print medium P is brought into tight contact with the outer surface of the fixing belt 203, and the print medium P is clamped and conveyed together with the fixing belt 203 through the fixing nip 206. During clamping and conveyance, heat of the fixing heater 204 is applied to the print medium P via the fixing belt 203, and the pressure of the fixing nip 206 is also applied. Accordingly, the unfixed toner image t is fixed as a permanently fixed image onto the print medium P by the heat and pressure. The print medium P passes through the fixing nip 206, self-strips from the surface of the fixing belt 203, and is discharged.
In the fixing unit 201 having this configuration, the heat capacity of the fixing belt 203 is very small, and the fixing nip 206 can reach a toner image fixable temperature within a short time after the fixing heater 204 is powered.
However, when the belt fixing unit 201 using the fixing belt 203 having no elastic layer is adopted as a fixing unit for a color image forming apparatus, the surface of the fixing belt 203 cannot follow undulations on the surface of the print medium P, undulations caused by the presence/absence of a toner layer, undulations of the toner layer itself, and the like because no elastic layer is formed on the fixing belt 203 serving as a fixing member. This results in a difference in heat amount applied from the fixing belt 203 between the recess and the projection. That is, the applied heat amount is large at the projection in good contact with the fixing belt 203 because heat conducts well from the fixing belt 203, but is small at the recess in poor contact with the fixing belt 203 because heat does not conduct well from the fixing belt 203 in comparison with the projection. Since the heat amount applied to the toner layer changes between the recess and projection of the toner layer, the toner fusing state becomes nonuniform and appears as gloss nonuniformity, adversing a fixed image.
Especially, a color image is formed by superposing and mixing toner images of a plurality of colors, and thus the toner layer is undulated more greatly than a monochrome image. When the fixing belt 203 does not have any elastic layer, gloss nonuniformity of a fixed image becomes more conspicuous, degrading the image quality. When the print medium P is an OHP sheet and an image is projected after fixing it, light scatters owing to a microscopically nonuniform surface of the fixed image, decreasing the transparency.
The fixing belt 203 may be coated with silicone oil or the like so as to sufficiently uniformly conduct heat to the fixing belt 203 having no elastic layer and the undulated portion of the print medium P or unfixed toner image t. This method increases the cost, and the fixed image and print medium P become oily.
To prevent these problems, there has been proposed a fixing unit which forms a low-cost color on-demand fixing unit by using a fixing belt with an elastic layer for a belt fixing unit (see Japanese Patent Laid-Open No. 11-15303).
FIG. 20 depicts a schematic view showing the schematic configuration of a belt fixing unit using as a fixing member a fixing belt 203 with an elastic layer. The same reference numerals as those in the device of FIG. 19 denote the same structuring members and parts, and a repetitive description thereof will be omitted.
In a heat roller fixing type fixing unit, the heat capacities of the heat roller and press roller 205 are large, and the temperature is simply kept at a predetermined value. On the contrary, a film or belt heating type fixing unit reduces the heat capacity in order to ensure an on-demand characteristic, and poses the following problems. More specifically, in the film or belt heating type fixing unit, the heat amount applied to the print medium P greatly changes depending on the temperature of the press roller 205, and the temperature of the press roller 205 greatly varies depending on the using state. It is therefore difficult to keep the heat amount applied to the print medium P uniform.
At this time, it is difficult to obtain good fixation and a uniform gloss value regardless of the using state. Depending on conditions, not only the gloss value greatly varies upon great variations in the temperature of the press roller 205, but also an image failure such as a fixing failure or hot offset occurs. Further, the print medium P may be wound around the fixing unit.
To solve these problems, a belt fixing type fixing unit which is often used in a monochrome image forming apparatus and does not have any elastic layer makes the following proposals.
As the first example, temperature detection units (not shown) are arranged for not only the heater but also the press roller 205, and detect the temperatures of both the fixing heater 204 and press roller 205. The heat accumulation amount in the press roller 205 is considered on occasion, and the fixing temperature is so determined as to maintain the heat amount applied to the print medium P at the fixing nip 206 at a predetermined reference value (see Japanese Patent Laid-Open No. 6-149102).
As the second example, the temperature detection unit 209 which abuts against the back surface of the fixing heater 204 is used. The fixing temperature is determined on the basis of a temperature detected by the temperature detection unit 209 before the start of energization and a change in temperature detected by the temperature detection unit 209 after the end of energization to the fixing heater 204 (see Japanese Patent Laid-Open Nos. 6-289750 and 11-194649 and Japanese Patent No. 3,244,838).
As the third example, there is proposed a method utilizing sheet count control in which the fixing temperature is determined on the basis of the number of fixed sheets. This method is characterized by switching the fixing temperature as the number of fixed sheets increases. There are proposed a method of determining an apparent number of sheets in accordance with a temperature detected by the temperature detection unit 209 immediately before printing, a method of counting an apparent number of sheets in intermittent printing more than in continuous printing, and a method of counting an apparent number of sheets in a different unit in accordance with the time till reception of the next print signal after energization to the fixing heater 204 stops or power is reduced (see Japanese Patent Laid-Open No. 2002-169407).
However, the following problems occur when the fixing temperature determination method used in a monochrome image forming apparatus is applied to a color image forming apparatus.
These problems will be explained.
For example, according to Japanese Patent Laid-Open No. 6-149102, a temperature detection unit arranged for the press roller 205 leads to a bulky apparatus and high cost. When the temperature detection unit 209 contacts the press roller 205, it contaminates the press roller 205 with toner, paper dust, or the like, and contaminates the print medium P when the print medium P passes. The temperature detection unit 209 damages the press roller 205 on standby, and forms a trace corresponding to the damage of the press roller 205 on an image in double-sided printing.
For example, according to Japanese Patent Laid-Open Nos. 6-289750 and 11-194649 and Japanese Patent No. 3,244,838, the fixing belt has an elastic layer, and the heat capacity of the fixing belt is large. Thus, the temperature difference between a temperature detected by a thermistor on the back surface of the heater and the temperature of the press roller 205 may become large after the end of energization to the fixing unit. The temperature of the press roller cannot be detected because the relaxation time until the temperature difference between a temperature detected by the thermistor on the back surface of the heater and the temperature of the press roller 205 after the end of energization to the fixing unit relaxes and these temperatures become almost equal to each other is much longer than that of a fixing unit using a fixing belt having no elastic layer. Since the temperature of the press roller cannot be detected, this method cannot be utilized in a color image fixing unit of, e.g., an electromagnetic induction heating type which is not equipped with any temperature detection unit near the fixing nip, e.g., on the back surface of the fixing heater. The color image fixing unit requires a means for completely indirectly predicting the temperature of the press roller.
For example, Japanese Patent Laid-Open No. 2002-169407 has the following problems in determining fixing temperature control on the basis of the number of fixed sheets.
A color image fixing unit generally uses a large amount (M/S) of toner to be fixed, and the fixing temperature is higher than that of a monochrome image. Also in a monochrome image forming apparatus, the fixing temperature rises as the speed increases.
For a high fixing temperature, power applied to start fixing must be increased to ensure the on-demand characteristic, and the heat amount applied to the press roller at the start of fixing more greatly increases. That is, the temperature of the press roller at the start more greatly rises. In a fixing unit used in the above-described color image forming apparatus, the fixing belt has an elastic layer, and the heat capacity of the fixing belt is large. Power applied to start fixing must be set large for a color image, and the temperature of the press roller further rises because the nip is kept heated until the fixing belt reaches a predetermined temperature.
After the start of feeding a transfer medium P, the transfer medium P intermittently deprives the press roller of heat, and the temperature of the press roller stabilizes at an almost constant temperature. When the speed of the fixing unit increases, an amount by which a print medium passes through the fixing unit per unit time increases. An amount by which the print medium deprives the press roller of heat also increases, and the temperature of the press roller hardly rises in continuous printing.
In other words, when a color fixing unit is used or higher-speed printing is done, the temperature rise rate of the press roller in continuous printing becomes much lower than that of the press roller at the start.
If the temperature rise rate of the press roller greatly changes in this way, the saturation temperature of the press roller after printing continues becomes much higher in intermittent printing than in continuous printing. When the printing speed becomes higher, the temperature of the press roller hardly rises and stays at a relatively low temperature in continuous printing, but greatly rises in intermittent printing.
In the use of the technique disclosed in Japanese Patent Laid-Open No. 2002-169407, the saturation temperature upon printing of a large number of sheets is different between continuous printing and intermittent printing in sheet count control, and thus the temperature of the press roller cannot be detected at high precision. That is, sheet count control which satisfies both continuous printing and intermittent printing cannot be achieved.
In Japanese Patent Laid-Open No. 2002-169407, a belt having no elastic layer is used, and the temperature of the press roller is measured by a thermistor which abuts against the fixing heater. On the contrary, in the use of a fixing belt having an elastic layer, the time until a temperature detected by the thermistor becomes equal to the temperature of the press roller is very long, and the temperature of the press roller cannot be detected in real time. Also, the temperature of the press roller cannot be detected in a color image fixing unit of, e.g., an electromagnetic induction heating type which is not equipped with any temperature detection unit near the fixing nip, e.g., on the back surface of the fixing heater. In this case, the temperature of the press roller cannot be obtained upon power-on/off, and no appropriate control temperature can be selected.
As described above, it is difficult for the conventional methods to obtain stable fixation and a uniform gloss value. Depending on conditions, the gloss value greatly varies, and an image failure such as a fixing failure or hot offset occurs. The conventional methods also suffer a technical problem in which the print medium P is wound around the fixing unit.